I highly doubt moving from Windows 7 to Windows 8 with the same CPU would make a drastic difference in temperatures, unless your power profile is different, or your hardware configuration is different, or if there is an external factor (ambient temperature) is different. Unless these temperatures were taken on the same day, I'd highly doubt it's Windows 8.
Your CPU won't downclock that far. It should go to about 800 MHz - try a different program to measure clock speeds.
uh, I just tried 4 programs, none of them say the clock speeds went down.
You might highly doubt that moving from Win7 to Win8 would change temperatures, but you'd be wrong. Someone else here already confirmed it. The hardware configuration is the exact same. My idle temperatures are incredibly consistent, and do not vary by more than 5 degrees over an entire year. My Win7 idle temp was 39-40*C, every day, for the past 3 months. Then one day, I installed Windows 8, and my idle temp was 33-34*C, on the very same day that Win7 measured 39-40*C, using the same measuring softwares (I used SpeedFan and OHM on both OSes), which both report that they are getting the data from the same internal sensor ID (IT8720F-1).
So the only explanations I can think of is that either A) my Win7 install had more CPU usage when idle (very possible, I had a ton of programs installed, but I checked CPU usage often and I would try to eliminate programs that used CPU in the background) than my Win8 install did. Since I am running the same set of programs on Win8 that I was on Win7, I have to believe that the Windows 8 kernel itself is less CPU-intensive. This would make sense, since they designed it to be run on battery-powered mobile devices.
or B) the CPU clock speed is being decreased when IDLE, but it isn't being accurately measured by any of the 4 programs I've tried. Even though they all report the CPU clock speed in real-time, and I can see the minor variations in 100's of kHz in clock speed, so I know that they aren't just a static snapshot of a CPU clock speed in the past.